This invention concerns new polyester filaments, having properties that make them especially suitable for use as a replacement for cellulose acetate in "flat" yarns and in continuous filament tows, and new polyester staple fiber, and their production.
Polyester continuous filaments have been prepared commercially for many years, and are now manufactured in very large quantities for use as continuous filament yarns and tows. The tow is generally crimped and converted to staple fiber which is drafted and twisted into "spun" yarns, or may be converted to staple for other uses, e.g. flock. Continuous filament polyester yarns are frequently textured to impart a "spun-like" tactility, usually by false-twist texturing, but may alternatively be used without texturing, in which case they are often referred to as "flat" yarns. Most commercial manufacture has been of poly(ethylene terephthalate) because of the physical properties and economic advantages of this synthetic filamentary material. Most of the commercial yarn is processed into fabrics for apparel purposes, and is therefore dyed at some stage.
It is well recognized, however, that poly(ethylene terephthalate) is more difficult to dye than other filamentary materials, such as cellulose acetate, so special dyeing techniques have been used commercially, e.g. dye bath additives called "carriers" have been used to dye the homopolymer, usually at higher pressures and temperatures, or the chemical nature of the polyester has been modified to increase the rate of dyeing, e.g. by introducing tetramethylene groups, or to introduce dye-receptive groups, e.g. as discussed in Griffing & Remington U.S. Pat. No. 3,018,272. These special techniques involve considerable expense, and it has long been desirable to provide poly(ethylene terephthalate) filaments having useful physical properties, e.g. for apparel and home furnishing applications, but which can be dyed at the boil (i.e., without requiring superatmospheric pressure and apparatus suitable for such pressure) within a reasonable period of time without carriers. Although all physical and chemical properties of textile yarns should be considered, the most important physical properties are generally the tensile and shrinkage properties.
The tensile properties of commercial (drawn) flat polyester yarns have been satisfactory for many textile purposes, and are generally of the approximate order: tenacity 4 grams per denier; elongation 30%; (initial) modulus 100 grams per denier in as-produced condition, but 50 to 65 grams per denier after boiling in a relaxed state. Although the elongation is usually given, the modulus is often of more significance in determining suitability for particular textile purposes. The high modulus of existing commercial polyester yarns has been considered important for many textile purposes. Cellulose acetate, however, has been preferred for other flat yarn end-uses, e.g. in taffeta and other closely-woven fabrics because of its lower modulus (of the order of 40 grams per denier), and correspondingly preferred tactile aesthetics. Existing commercial polyester flat (drawn) yarns have a modulus that is too high for such polyester yarns to be preferred over cellulose acetate in such end-uses. Cellulose acetate, however, has inferior tenacity, especially when wet.
For most consumer purposes, a commercial flat yarn should have a low boil-off shrinkage. Hitherto, it has been customary to prepare fabrics with commercial polyester flat yarns of boil-off shrinkage about 8 to 10%, and then reduce the boil-off shrinkage by heat-setting the fabric. Even when existing commercial polyester textile yarns are heat-set, they are not stabilized against shrinkage at temperatures higher than the temperature of heat-setting, because a characteristic of these (drawn) polyester yarns is that the shrinkage increases significantly with increasing temperature. Thus, prior commercial polyester yarns have not been truly thermally dimensionally stable in the same sense, for instance, as a cellulose acetate yarn, whose shrinkage does not increase significantly with temperature. It would be desirable to provide polyester yarns that, after being boiled off, would not shrink significantly, so that heat-setting would be unnecessary to avoid shrinkage during fabric finishing. A low shrinkage tension is also desirable when finishing.
As indicated, some of the properties (such as modulus) of prior polyester yarns have differed according to whether the yarn has been in as-produced condition or has been shrunk, which latter condition is termed herein "after boil-off shrinkage." Properties under both conditions can be important. The yarn manufacturer and textile processor is mainly concerned with the properties in as-produced condition until the yarn is boiled, generally when the fabric is scoured and/or dyed, whereas the ultimate consumer is concerned with the properties of the shrunk fabric, i.e. after boil-off shrinkage. Hitherto, polyester yarns have not been manufactured commercially with properties such that the modulus of the as-produced yarn is of the same order as the modulus of the yarn after boil-off shrinkage.
When dyeing commercial drawn poly(ethylene terephthalate) yarns, dyeing defects result largely from lack of physical uniformity in the yarns. Such defects are more noticeable when dyeing at the boil (i.e. at atmospheric pressure), but use of higher pressures with carriers can give more uniform dyeing. Dyeing defects are readily apparent in taffetas, and other closely woven fabrics, in which flat yarns are used. Uniformity can be of critical importance for apparel yarns. In a customer's opinion, it is probably the most important characteristic. Any polyester replacement for cellulose acetate must dye uniformly, if it is to succeed, and this means that the polyester must show good physical uniformity, as discussed hereinafter.
Thus, it would have been very desirable for certain end-uses to provide poly(ethylene terephthalate) flat yarn with desirable tensile properties, including a suitable lower modulus, a modulus that is not significantly different after boil-off shrinkage, low boil-off shrinkage, thermal stability, and better dyeing properties, but such a combination has not previously been available commercially. It would also be economically desirable to prepare useful continuous filaments for such yarns or tows directly in the as-produced condition, so that flat continuous filament yarns, for example, would need no further processing in the nature of drawing and annealing but could be used directly to prepare fabric.
For many years, polyester filaments were melt spun and withdrawn from the spinneret at relatively low speeds of up to about 1000 meters/minute. These low speed spun undrawn filaments were then subjected to a separate drawing operation, either after winding up the low speed spun filaments in a "split" process, or in a "coupled" continuous operation in which the filaments were first withdrawn at a relatively low speed (less than 1000 meters/minute) and then subjected to drawing without intermediate windup. Hitherto, drawing has been a step in the commercial manufacture of all flat polyester textile yarns.
More recently, polyester filaments have been prepared commercially on a large scale by high speed spinning on windups that are capable of operation at speeds up to about 4000 meters/minute, e.g. as supplied by Barmag Barmer Maschinenfabrik AG, and being described, for instance, in a brochure entitled "SW4S SW4R Spin Draw Machines" and published about June, 1973. The polyester filaments commercially produced at such speeds are referred to as "partially oriented" and have been particularly useful as feed yarns for draw-texturing, as disclosed by Petrille in U.S. Pat. No. 3,771,307. These yarns have not been useful as flat yarns. Their tenacity and modulus have been lower, while their elongation and shrinkage have been higher, than commercial polyester flat yarns; their shrinkage has generally been at least 60%, i.e. much too high for normal textile purposes. The subsequent draw-texturing operation raises the tenacity and modulus and reduces the elongation and shrinkage to the values that have hitherto been considered desirable for polyester textile yarns.
Thus, heretofore, drawing has been a step in all commercial manufacture of polyester textile yarns.
High speed spinning of polyester filaments at speeds of 3000 to 5200 yards per minute was suggested 25 years ago by Hebeler in U.S. Pat. No. 2,604,689, with the objective of providing wool-like yarns of low modulus 10 to 50 grams/denier (110 to 550 kg/mm.sup.2). Spinning at even higher speeds, above 5200 yards per minute, was suggested by Hebeler in U.S. Pat. No. 2,604,667 with the statement that lower spinning speeds result in high shrinkage yarn of quite different properties. High speed spinning generally, has received much attention, e.g. by H. Ludewig in Section 5.4.1 of his book "Polyester Fibres, Chemistry & Technology," German Edition 1964 by Akademie Verlag and English translation 1971 by John Wiley & Sons, Ltd., and the effect on shrinkage is discussed in Section 5.4.2. More recently, there has been interest in high speed spinning at speeds much greater than 4000 meters/minute, e.g. as disclosed by F. Fourne in Chemiefasern/Textil-Industrie, Dec. 1976, pages 1098-1102, the emphasis being on providing continuous filament yarns and tows (for staple fiber) by spinning at these much higher speeds, using faster windups, rather than at speeds of the order of 4000 meters/minute, using prior windups. It would be desirable, however, to provide useful continuous polyester filaments, as indicated hereinbefore, using existing commercial windups operating at about 4000 meters/minute, rather than these much greater speeds, because of the cost of developing and operating the latter.
There has also been recent interest in spinning at speeds of about 4,000 meters/minute and in modifying the process conditions to reduce the shrinkage of the resulting filaments. For instance, E. Liska in Chemiefasern/Textil-Industrie, Sept. 1973, pages 818-821, Oct. 1973, pages 964-975 and Nov. 1963, pages 1109-1114, discusses the structural changes in polyester fibers from orientation (obtained by high speed spinning) and annealing, and recommends raising the molecular weight (intrinsic viscosity) and the denier per filament to reduce the shrinkage. Raising the viscosity has also been suggested, e.g. in Japanese Patent Publication No. 49-80322/1974 (Kuraray Company). This is costly and is not desirable for apparel yarns as a cellulose acetate replacement.
So far as is known, it has not previously been suggested that the problem of producing a poly(ethylene terephthalate) flat yarn having an acceptable combination of dyeability (superior to that of commercial (drawn) flat poly(ethylene terephthalate) yarns) and of physical properties, especially tensile properties and thermal dimensional stability, could have been solved by spinning directly poly(ethylene terephthalate) filaments having such superior dyeability and acceptable physical properties, using windups capable of operation at about 4000 meters/minute.